Don't pass pgpassword to towel.

[novotl]

Summary

Closes #315

Without the rest of the PG connection string ({{ include "prefect-server.postgres-connstr" . }}) the password seems useless.

Compare it for example to hasura that receives both the connection string and the password

server/helm/prefect-server/templates/hasura/deployment.yaml

Lines 50 to 54 in e884510

	- name: HASURA_GRAPHQL_DATABASE_URL
	value: {{ include "prefect-server.postgres-connstr" . }}
	- name: PGPASSWORD
	valueFrom:
	{{- include "prefect-server.postgres-secret-ref" . | nindent 16 }}

Importance

Checklist

This PR:

• successfully rendering the template with helm template helm/prefect-server
• adds new tests (if appropriate)
• adds a change file in the changes/ directory (if appropriate)

[zanieb]

Thanks for contributing!